This invention relates to detectors of optical radiation (i.e., lightwaves) and, more particularly, to semiconductor photodiodes.
The recent special issue of the Western Electric Engineer, Vol. XXIV, No. 1, Winter 1980, is a graphic illustration of the burgeoning interest in lightwave communications systems, especially fiber optic systems. The rapid growth of these systems has engendered commensurate activity in optical sources and detectors, primarily GaAs-AlGaAs laser diodes and LEDs in conjunction with Si APDs and p-i-n diodes for present applications at relatively short wavelengths (e.g., 0.80-0.90 .mu.m), and InP-InGaAsP laser diodes and photodiodes for future systems at longer wavelengths (e.g., 1.1-1.6 .mu.m).
In general, a photodiode operates by the absorption of light which generates electron-hole pairs in the depletion region of a p-n junction. In the photovoltaic mode or under reverse bias, the junction field separates the pairs and thereby produces a photocurrent which can be made to do useful work in an external load. The optical-to-electrical conversion efficiency can be enhanced by employing a p-i-n photodiode configuration in which the impurity concentration of the i-layer is low enough to produce complete depletion. A depleted i-layer, often called the active layer where light is primarily absorbed, means that pairs can be readily separated and do not recombine before producing a useful photocurrent. Although the i-layer should be made thick enough to absorb a substantial fraction of the light incident thereon, it is often made even thicker to reduce leakage current, increase the reverse breakdown voltage, and lower the capacitance of the photodiode. On the other hand, the maximum thickness of the i-layer is limited primarily by the required speed of operation.
Realizing low-doped i-layers can often be a problem depending on the materials from which the photodiode is made and the fabrication techniques employed. For example, suitable p-i-n photodiodes can be made of silicon using, inter alia, high resistivity (&gt;300 .OMEGA.-cm) epitaxial layers and ion-implantation (see, U.S. Pat. No. 4,127,932 granted to A. R. Hartman et al), yet the ability to controllably fabricate similar devices from Group III-V compound semiconductors is complicated by the difficulty of making low-doped material (e.g., 10.sup.15 cm.sup.-3). This problem in turn limits the maximum depletion width attainable and hence places constraints on desired levels of leakage current, breakdown voltage, and capacitance. There is a need, therefore, to be able to fabricate relatively wide (e.g., 10 .mu.m) depletion layers in more highly doped (e.g., mid-10.sup.16 cm.sup.-3) Group III-V compound semiconductors.
The fabrication of prior art photodiodes is also disadvantageous because of the need to grow a plurality of epitaxial layers of controlled composition, conductivity type and thickness often involving sophisticated growth procedures (e.g., LPE, MBE, VPE) and a complicated sequence of ion-implantation and/or diffusion steps. So, there is also a need to simplify the fabrication of photodiodes and, thereby, to increase reproducibility and reduce costs.